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Full text of "Text Book Of Mechanical Engineering"

76                               Puddled Bar.

into balls or blooms, and these are then removed and compressed
under a steam hammer by rapid blows, so as to squeeze out the
slag. The blooms are next rolled out and further squeezed by
being passed through the rolls of a rolling mill, giving us iron
called Puddled Bar.

These bars are now broken up and re-worked by hammering
and rolling, more or less, depending on the degree of purity and
strength which is required, and we thus have the varieties of
wrought iron known asócommon, best, double best, and treble best,
which are used for various ordinary forgings, while Low Moor iron
is required for the fire-boxes of steam boilers and for more
difficult forgings.

The purification of the iron obtained in a puddled bar is
shewn by the following table, which may be compared with the
table showing the composition of white pig (p. 2) :ó

Table showing chemical composition  of Puddled Bar, in
percentages.

Iron         .........        ......        99*31

Combined Carbon         ...        ...        ...           -3

Silicon     ...        ...        ...        ...        ...           '12

Sulphur   ...        ...        ...        ...        ...           -13

Phosphorus        ...        ...        ...        ...           '14

lOO'OO

Wrought Iron during its conversion from the pig, has lost
the capability of being cast Into moulds, but has acquired a new
nature, becoming viscous or sticky, and, as a result, maybe worked
by the smith, when white or red hot, its formation into different
shapes being assisted by the property of 'welding, which as cast
iron it did not possess. Repeated rolling gives a fibrous quality,
making the iron both stronger and more homogeneous or uniform
in texture, and these fibres may be seen on breaking a bar of
rolled iron, which then has the appearance shewn at A, Fig. 86,
while cast iron or even puddled bar gives a granular fracture (B).

Rolling or hammering iron when cold or nearly so gives it a
crystalline structure near the surface, so that T iron is not so
strong as bar iron, and plates still weaker. Re-heating and slow
cooling tends to remove this source of danger. (Seep. 1013.)